Mineralization and ore genesis of the Qiaoxiahala Fe-Cu-(Au) deposit in the northern margin of East Junggar terrane, Central Asian Orogenic Belt: Constraints from fluid inclusions and stable isotopes

Abstract

The Qiaoxiahala Fe-Cu-(Au) deposit (1.44Mt ore @ 43–53% Fe, 0.55–2.21% Cu and 0.13–2.4g/t Au) is located in the northern margin of East Junggar terrane, NW China. The deposit is hosted in the upper part of the Middle Devonian Beitashan Formation mafic and felsic volcanic rocks and sedimentary interbeds, and is spatially closely related to local fractures. Six hypogene hydrothermal stages have been identified, including (Stage I) early skarn, (Stage II) late skarn, (Stage III) magnetite mineralization, (Stage IV) magnetite-pyrite mineralization, (Stage V) chalcopyrite mineralization and (Stage VI) late veins. These six stages include three major mineralization events, as represented by the (Stage III) magnetite-epidote-K-feldspar-quartz-calcite, (Stage IV) magnetite-pyrite-garnet-quartz-calcite and (Stage V) chalcopyrite-chlorite assemblages. The Stage III magnetite mineralization fluids were likely of high temperature (354–386°C), low-medium salinity (9.9–14.7wt% NaCl equiv.), Mg/Fe-rich and organic-rich with lithostatic pressure of 1.3–1.5kbar and corresponding depth of 4.3–4.8km.Oxygen and hydrogen isotopes showed that the 18O-rich (9.7–11.2‰) and D-rich (−26.3‰) fluids were possibly modified from the Early Carboniferous seawater interacting with the Beitashan Formation organic-rich limestone or tuff. In comparison, the Stage IV magnetite-pyrite mineralization fluids were likely of medium-high temperature (272−453°C), low-medium salinity (2.9–19.5wt% NaCl equiv.), Mg/Fe-rich, with pressure of 0.8–1.2kbar and corresponding depth of 2.6–4.0km. The Stage V chalcopyrite mineralization fluids were likely of low-medium temperature (186−270°C), low-medium salinity (1.4–21.8wt% NaCl equiv.), Ca- or Na-rich, with pressure of 0.4–0.6kbar and corresponding depth of 1–2km. Hydrogen and oxygen isotopic compositions (Stage IV: δDwater=−141 to −92‰; δ18Owater=8.9–11.1‰; Stage V: δDwater=−104‰; δ18Owater=7.2‰) suggest that the ore fluids were of connate marine origin. Furthermore, Stage IV δ13Cwater values (−0.6 to 0‰) also indicate interactions between seawater and sedimentary wall-rocks. The negative sulfur isotopic compositions of chalcopyrite and pyrite (−4.2 to −0.2‰) may demonstrate a biogenic sulfur source.Compared with the arc-related IOCG deposits in the central Andean orogen, the Qiaoxiahala Fe-Cu-(Au) deposit shares many common features in geology, mineralization, alteration and various sources of fluids, indicating a possible IOCG-like mineralization-style and potential to discover other Late Paleozoic arc-related IOCG deposits in East Junggar.